在固定时间内在排序列表中查找索引

Question

I have a set of custom objects I'm adding to a list and then sorting with a custom comparator. I then have a second list of some subset of these objects which I need the index of in the first list so that I can find the objects which are equal (as defined by my comparator). As the list is sorted these objects should simply be the objects directly before or after this object, but I don't know beforehand how many will be the same (likely 2-3).

I need this lookup to be in constant time as my list of sorted objects will be quite large. I could obviously just use list.index(), but that will be O(N) and I think I can do better. My first thought was to use a doubly linked list, but it looks like I would need to implement that myself and I'm not exactly sure how I would sort it.

Is there a double linked list implementation in python? Or is there a better alternative to this problem? Also, I'm currently on python2.5 and can't update my version, but if I'm limited by my version I'm still interested in hearing about that solution.

Answer 1

For the price of additional O(N) storage, you might create a dictionary to back the list indices of the sorted list:

index_map = dict((e,i) for (i,e) in enumerate(sorted_list))

This would give you O(1) lookup for the position of items in your second list.

If the elements of sorted_list are custom objects, then, as @mseifert points out, this approach relies on the objects having __eq__ defined properly, and also being hashable.